Recently, the tendency of using Light-Emitting Diodes (LEDs) as light source for electronic devices, lighting devices, etc. is continuously increasing. However, in order to completely replace with LEDs the traditional light sources, especially the indoor lighting devices, the intensity of the light emitted by the LEDs must be greatly enhanced.
There are LEDs with different nominal power. The nominal power is the electric power with which the LEDs shall be driven. The nominal power is limited. If a LED is driven with an electric power larger than the nominal power the increase in brightness is lower than below the nominal power. This effect is well known and to minimize this effect it is known to actively cool the LEDs. However, an active cooling is very laborious and in most applications of LEDs it is not possible to provide a suitable cooling mechanism. If the breakdown state is achieved the brightness can not be further increased even if a higher electric power is applied. The breakdown state is usually achieved with a current of more than about 40% to 50% of the nominal constant current. For example, if the nominal constant current is about 350 mA then the breakdowns state is achieved by about 500 mA.
The standard LEDs have a nominal power of about 1.2 W. There are also known high power LEDs with a nominal power of about 3 W or 5 W. A nominal constant current corresponds to a nominal power as the LEDs are always driven with a voltage of 3.5 V. So the nominal power is the nominal constant current multiplied with 3.5 V.
U.S. Pat. No. 6,028,694 discloses an illumination device using a pulse modulation technique for providing an increased light output for a given heat load. This illumination device is designed for being used in surgery applications. The power is supplied in pulses to periodically activate a short wavelength emitting LED. These light pulses are stimulating a phosphor-based color conversion system to produce white light. The light pulse from the LED briefly excites the phosphor system, producing a bright illumination during the interval while power is dissipated in the LED, the LED warms. After the pulse ends and before the next pulse begins, the LED cools because no more power is dissipated in the LED. The intensity of the illumination produced by the phosphor gradually decays between the light pulses. However, the average illumination produced over the entire period is higher than a conventional LED illumination device using constant power dissipation for a given heat load. As this device is designed to be used in a human body it is only driven with a low electric power, because otherwise the human body would be injured due to the created heat.